This invention relates to a magnetic recording medium, and more particularly to a perpendicular magnetic recording medium which includes a magnetic recording layer having an axis of easy magnetization in the direction essentially perpendicular to layer surface thereof.
With recent advances in information processing, the quantity of information written onto and read from memory devices has been increasing rapidly. The demand for increased capacity for magnetic recording media such as floppy disks has also been increasing.
In order to respond to such a demand, active research and development have been pursued on magnetic recording media with high density capability, in particular on perpendicular recording media which realize magnetic data storage by establishing residual magnetism in the direction essentially perpendicular to the layer surface. The magnetic recording medium for perpendicular magnetic recording a magnetic recording layer has an axis of easy magnetization in the direction essentially perpendicular to the surface thereof. As such a magnetic recording layer, there is known a metallic film medium formed by sputtering or vapor deposition of a metallic film such as a Co-Cr system alloy or the like, or an oxide film medium with the magnetoplumbite crystal structure such as Ba ferrite or Sr ferrite. Such a perpendicular magnetic recording medium is considered to be promising as a medium better suited for high density recording than a particular medium which is employed for the majority of the longitudinal magnetic recording media.
In a particular medium, the magnetic recording medium is formed by coating a base (substrate) with a magnetic powder mixed with a binder or the like. The magnetic recording layer thus formed is elastic. In addition, it is possible to mix a lubricant in the magnetic layer. This helps to maintain an intimate contact between the magnetic medium and a magnetic head for writing on/reading from the magnetic medium, making it possible to give a sufficient durability to the medium and the head.
In contrast to this, a magnetic recording layer of the metallic film medium or the oxide film medium has little elasticity. When a magnetic head made of a hard material such as ferrite runs on a magnetic medium, damages like scratches tend to be created on the surface of the medium or the head. In that case, not only is the durability of the medium and the head reduced, but also the effective spacing between the medium and the head is increased due to adherence of the powder abraded from the medium or the head. This results in an increase in what is known as "spacing loss" which becomes the cause of degradation in the frequency characteristic and a lowering of the variations in the reproduced output level.
For this reason, it is considered, in the case of the metallic film and the oxide film medium, to form a protective layer made of a hard material over the magnetic recording medium to protect it. As a concrete example of the protective layer, there has been proposed a film of silicon oxide, aluminum oxide, titanium nitride, and others. However, the quality of material of such a protective layer is brittle so that it tends to be worn out by its contact with the magnetic head. Then, the resulting worn powder damages the medium and the head, showing that it will not be sufficient as a solution to the above problem.
In this case, forming a protective layer thick enough may be considered to be effective for preventing the abrasion of the magnetic recording layer. However, it is not desirable from the viewpoint of the perpendicular magnetic recording characteristic. Namely, the recording density of the perpendicular magnetic recording system is essentially much higher than that of the longitudinal magnetic recording system, and the recording wavelength may be reduced. It becomes necessary to restrict the effective spacing between the head and the medium to an extremely small value, and accordingly, the thickness of the protective layer is limited also. When the thickness of the protective layer is restricted in this way to the extent that it does not injure the perpendicular magnetic recording characteristic, the durability improvement is inadequate.
As an alternative there may be considered a method of coating a lubricant over the magnetic recording layer. However, in the case of a metallic film type medium, the film formed by sputtering or a like method has such a smooth surface that the wetting and holding power for a lubricant are low. Therefore, it is difficult to spread a lubricant over the layer with sufficient adhesive power and uniformity so that the above problems still remains unsolved.